Battery specs (kWh, kW, DoD), cycling, warranties
Key battery specifications that affect performance and cost:
- Capacity (kWh): Total energy storage. Typical residential batteries: 10–20 kWh. Larger capacity stores more energy but costs more.
- Power (kW): Maximum discharge rate. Typical residential batteries: 5–10 kW. Higher power supports more appliances during backup.
- Depth of discharge (DoD): Usable capacity (typically 80–90%). Higher DoD means more usable energy but may reduce battery life.
- Cycling: Number of charge/discharge cycles. Typical warranties: 4,000–10,000 cycles or 10 years. More cycles mean longer battery life.
- Warranties: Product warranty (typically 10 years) and performance warranty (guarantees minimum capacity after warranty period).
Compare battery specifications and warranties across options. Higher capacity and power cost more but offer better backup and TOU savings.
Backup circuits vs whole-home; transfer switches; outage behavior
Battery backup systems can power your entire home or selected circuits:
- Backup circuits: Power selected circuits (lights, fridge, outlets) during outages. Lower cost but limited backup.
- Whole-home backup: Power entire home during outages. Higher cost but full backup. Requires larger battery capacity.
- Transfer switches: Automatically switch from grid to battery during outages. Required for backup systems.
- Outage behavior: Batteries can provide backup power during outages if charged. Solar can recharge batteries during outages if sun is available.
Backup circuits are more affordable and may be sufficient for essential loads. Whole-home backup requires larger batteries and higher cost.
TOU arbitrage and self-consumption
Batteries can improve savings under time-of-use rates:
- TOU arbitrage: Store solar during off-peak hours (low rates), use during peak hours (high rates). Can improve savings if rate differential is significant.
- Self-consumption: Use stored solar energy in the evening instead of buying from the grid. Reduces grid purchases and improves savings.
- Export shaping: Store excess solar instead of exporting at low rates, use stored energy when rates are higher.
TOU arbitrage works best if your utility has significant rate differentials between peak and off-peak hours. Check your utility's TOU rates and export credits.
Sizing a battery: goals and constraints
Battery sizing depends on your goals:
- Backup power: Size based on essential loads and desired backup duration. Typical backup: 10–20 kWh for 1–2 days of essential loads.
- TOU savings: Size based on peak-hour usage and rate differential. Typical TOU: 10–15 kWh to shift peak-hour loads.
- Self-consumption: Size based on evening usage and solar production. Typical self-consumption: 10–15 kWh to cover evening loads.
- Budget: Batteries add significant cost ($10,000–$20,000+). Consider ROI and whether backup or TOU savings justify cost.
Work with your installer to size batteries based on your goals, usage, and budget. Larger batteries cost more but offer better backup and TOU savings.
FAQ
Do I need batteries with solar
Batteries are optional. They add backup power and can improve TOU savings but increase cost. Most homes don't need batteries unless you want backup or have significant TOU rate differentials.
How long will batteries last during outages
Backup duration depends on battery capacity and load. Typical 10–20 kWh battery can power essential loads for 1–2 days. Solar can recharge batteries during outages if sun is available.
Will batteries always improve savings
Not always. Batteries add cost and may not improve savings if your utility has low TOU rate differentials or high export credits. Compare battery cost to potential savings.